In keeping up with recent IT evolution and with enhancement of the field of application of IT, the amount of data handled by an information processing system is increasing from day to day. These data are held in storages, and the amount of data held per storage is increasing. However, there is a limit to the data capacity that may be stored in each storage. To store data in excess of the capacity in the storage, it is necessary to increase the number of storages.
There are also cases where the limit of the performance of a storage is reached not due to the data capacity in the storage, but due to increase in the number of hosts (devices accessing the storage, such as client or server) attached to the storage. In this case, storage capacity enlargement or load distribution may be achieved through exchange of pre-existing storages (migration source storages) for newly extended storages (migration destination storages), or storage extension combined with distributed data storage.
To enable any one of plural hosts to access any one of plural storages, newly installed on extension, it is necessary to interconnect plural hosts and plural storages over a network. At present, it is customary to use an interconnection configuration in which plural storages of SAN (Storage Area Network) are interconnected over a network, dedicated to storages, to provide for reciprocal accessing to plural storages. To interconnect the storages of SAN, for instance, over a network, to enable shared use, an interface, such as a fiber channel, a network or a protocol, dedicated to storages, is used. Recently, the iSCSI (Internet Small Computer System Interface), implemented on TCP/IP, a general-purpose network protocol, is used as a new protocol for use between the hosts and the storage. The iSCSI layer is located, as a layered model, between the SCSI layer and the TCP/IP layer, receives an SCSI command, data and the response from the SCSI layer, capsules them to formulate an iSCSI PDU (Protocol Data Unit), and sends it via the TCP (Transport Control Protocol) connection of TCP/IP. The iSCSI layer also extracts the SCSI command, data and the response from the iSCSI PDU, received through the TCP connection, to deliver the so extracted command, data and response to the SCSI layer.
In an environment where plural storages are connected to plural hosts over a network, as in the SAN environment, it is necessary to change the storages as the destination for data retention, with a view to expanded storage capacity and to load distribution, thereby achieving more flexible system management. In an environment having many storages, data migration among plural storages (from the migration source storage to the migration destination storage) is implemented mainly by the following two methods:
The first method is to carry out data migration to a migration destination storage as an access to the migration source storage (processing on storage data, such as Read/Write) is inhibited. After completion of the data migration, the destination of connection of the host is switched to the migration destination storage to re-start the access.
The second method is to duplicate data in the migration destination storage, as the access from the host is continued, at the same time that the migration destination storage is updated and the destination storage of host connection is switched from the host with the updated area as a difference. The connection to the migration source storage is switched to the connection to the migration destination storage as the access from the host is inhibited and as the host side connection information is changed to establish a connection path, with the coincidence of duplication on the migration source storage with the duplication on the migration destination storage as a turning point.
As in the method for data migration, described above, it is necessary to switch the destination of connection from the migration source storage to the migration destination storage to carry out data migration among plural storages. The connection to be switched to the migration destination is the connection for a host to access data which has become the object for switching. Hence, the connection of the connection destination that needs to be switched is determined in terms of the data access unit or a data management unit in the storage of migration destination
Connection from the host to the storage over a network is via one of plural ports which is a network interface on the storage. To respective ports are allocated addresses of one or more networks.
A storage sets an interface for a host on a storage access protocol, termed a ‘target’ for one or plural addresses.
In the case of the iSCSI protocol, for instance, one or more IP addresses are allocated to each target. Logical connection and access from the host to the storage are carried out with the target defined on the storage as a unit. Each target corresponds to one or more memory areas in a storage termed as a ‘volume’.
Hence, in case of migrating a target on the migration source storage to the migration destination storage, it is necessary to migrate the volume corresponding to the target and the connection for the target.
In the above-described migration methods for storage data, switching of the destination of connection of the host has to be done in both cases as the access from the host is inhibited. That is, if this method for migration is used, it is necessary to halt the access from the host to the storage during the migration operation. Thus, if the access to the host side storage cannot be halted, some other technique needs to be used.
Among the means for switching the host connection destination storages without halting the access from the host, there are the following techniques, for instance.
First, in a SAN environment, if iSCSI is used for connection, the hand-over technique on the pre-existing TCP protocol may be used to provide for connection switching without halting the access from the host.
Transparent switching, as seen from the host of the connection path, may be implemented by the TCP handover technique. With the aforementioned protocol dedicated to storages, the transparent switching, as seen from the host of the connection path, cannot be implemented except if the OS on the host side or the driver is changed.
As an example of the technique for effecting the handover on the TCP protocol, there is a TCP migration technique of switching to different devices from one communication connection unit of the TCP protocol to another, as shown for example in the Non-Patent Document 1.
There has also been proposed a technique in which the storage of connection destination is switched and data is migrated without halting the accessing on the host side. This technique, however, applies only for a storage. In Patent Document 1 (JP patent Kokai Publication No. JP-P2001-249853A), there is disclosed a technique in which a special switch is installed in a network interconnecting the host and the storage to connect the migration source storage and the migration destination storage to the switch. In this technique, connection is transparently switched, from the migration source storage to the migration destination storage, within the inside of the switch, as two targets appear to the host at all times to be a sole target, by the action of the switch.
[Non-Patent Document 1]
Fine-Grained Failover Using Connection Migration Internet <URL: http://nms.lcs.mit.edu/papers/migrate-failover.pdf>
[Non-Patent Document 2]
Masahiko Takahashi and Tomoyoshi Sugawara, “Implementation of TCP non-connection process migration suited for data center environment,” Information Processing Society of Japan, OS Study Group, Jun. 17, 2004
[Patent Document 1]
JP Patent Kokai Publication No. JP-P2001-249853A